In modern vehicles, there are many functions that are controlled electronically. Some of these functions are of the on/off type, some can be switched to several positions and some are analogue. Directly coupled switches and sensors control most functions, but some functions require a contact-free (contactless) operation. An example of functions where contact-free operation is preferred includes ABS-sensors (ABS=Automatic Brake System), chassis height detection or switches that are exposed to weather, pollution and direct friction. One kind of contact-free switches and sensors are based on a magnetic principle. There exist different types of magnetic detectors such as reed-contacts, hall-sensors and other kinds of integrated magnetic detectors. A magnetic field is used to influence the detector. The detector and the magnet thus form the switch or the sensor.
To obtain a switch or a sensor with a high resolution and which at the same time is insensitive to external magnetic fields, it is desirable to position the magnet and the detector close to each other. In this way, it is possible to use a detector with a low sensitivity, obtaining a switch or a sensor that is not influenced by (insensible to) external magnetic fields.
One problem with magnetic switches and sensors is that the sensitivity of the detector must increase with an increased detection distance. For some applications, especially for magnetic switches, it may be possible to overcome the increased distance with a larger or stronger magnet having a stronger magnetic field.
A problem with the detector being very sensitive is that it will more easily be disturbed by an external, interfering magnetic field. This can, for instance, occur when the sensor is close to a high current cable or a large transformer. Thus, it is preferred not to raise the sensitivity too much for the detector.
A problem that arises when the magnetic field is increased by using a larger magnet is that the magnetic field is not only stronger, it is also more distributed in space. This gives the effect that, when an analogue detector is used, the resolution will be degraded due to the imprecise magnetic field.
Due to the nature and to the production process of permanent magnets, the magnetic properties for magnets can vary considerably, even if they are manufactured in the same batch and at the same time. Properties of the magnet that can vary include the magnetic remanence and the direction of the magnetic field. These varying properties in turn can cause magnetic switches and sensors to behave differently even if the specifications are equal. In production, this can cause considerable problems with making adjustments and the number of rejected parts (defects).